1. Field of the Invention
This invention relates to a catalyst for purifying exhaust gases emitted from an internal combustion engine or a boiler and a method for purifying the exhaust gas by the use of this catalyst. More specifically, it relates to an exhaust gas purifying catalyst capable of removing in high efficiency nitrogen oxides (NOx) in the lean atmosphere containing moisture and a method for purifying the NOx by the use of this catalyst.
2. Description of Related Art
The NOx emitted from internal combustion engines is a causal substance for acid precipitation and photochemical smog. In recent years, developing a means for decreasing NOx emission has been desired by the social consensus voiced about the environmental preservation. Though the purification or decomposition of NOx under an oxygen-excess atmosphere by catalytic method has been energetically studied since a long time ago, it has not been easy to discover an effective technique.
Titania-carried vanadium catalysts using ammonia as a reducing agent have been established as the technique for purifying NOx. It has been already put to practical use such as in large stationary sources for generation, but the effort to mount the systems has not yet been realized in mobile sources such as diesel cars on account of issues like slipping of unreacted ammonia.
In the circumstance, it was found to be effective for NOx purification to apply copper ion-exchanged zeolite using hydrocarbon as a reducing agent. With this discovery, zeolite based catalyst systems have been proposed in JP-A-63-100919 and JP-A-4-363146.
These catalysts, however, have entailed problems such as being still low selectivity at steady state or readily incurring deterioration under the influence of moisture.
This invention has been done in view of the affairs mentioned above and provides a catalyst capable of purifying, with high efficiency, NOx contained in the lean atmosphere in the presence of a hydrocarbon, and a method for the purification of NOx in exhaust gases by the use of the catalyst.
The object is accomplished by a catalyst with the composition formed by an intimate contact of zeolite and an oxide made from the complex of zirconium and manganese and/or cobalt in particularly like physical mixing.
The object is further accomplished by a method for the purification of NOx in exhaust gases by the use of the catalyst.
According to this invention, it is possible to provide a catalyst with the composition formed by an intimate contact of zeolite and an oxide made from the complex of zirconium and manganese and/or cobalt in particularly like physical mixing and that is capable of efficiently removing NOx in an oxygen-excess atmosphere containing moisture.
The above and other objects, features and advantages of the present invention will become clear from the following description of the preferred embodiments.